In the field of display devices such as a liquid crystal display, there is a demand for improved front side brightness. Therefore, in a backlight device for use in such a display, an optical member used to control the angular distribution of brightness and improve the front side brightness is provided. As disclosed by JP 3262230 B, a prism sheet is generally used as such an optical member.
As shown in FIG. 24, the prism sheet 100 has a plurality of prisms PL arranged in parallel to one another. Diffused light R100 from a surface light source is refracted at the side plane BP0 of a prism PL and emitted as it is deflected to the front surface direction. In this way, the prism sheet 100 improves the front side brightness of the display by deflecting the diffused light to the front surface direction.
However, the prism sheet 100 can improve the front side brightness while it also raises the brightness in the front side oblique direction. FIG. 25 shows the angular distribution of brightness for the vertical viewing angle in the prism sheet 100 having the prisms PL arranged in parallel in the vertical direction of the display screen. With reference to FIG. 25, the relative brightness is raised for vertical viewing angles in the range of ±30° using the prism sheet 100, while so-called side lobes whose relative brightness values peak around viewing angles of 80° and −80° in oblique directions to the front surface are also generated. Therefore, light that forms the side lobes (hereinafter referred to as “side lobe light”) does not contribute to improvement of the front side brightness. The prism sheet 100 cannot sufficiently collect outgoing light in the front surface direction and cannot reduce the side lobe light, and therefore the improvement of the surface side brightness has its limit.
Furthermore, a prism PL has a triangular cross section and therefore can easily be damaged during its manufacture, transport and installment to a backlight device, particularly at its apex. Such a defect is likely to result in a bright point or a dark point on the display. In order to prevent such defects, the prism sheet 100 before being installed into a display device must be provided with a protection film.